Devices such as press controllers must provide a high degree of safety and must be fail-safe so that they are switched to a safety side when failures, short circuits, disconnections, etc., occur. Load driving circuits for driving loads such as motors and solenoids that are controlled must also be fail-safe.
One of the conventional load driving circuits directly connects a semiconductor switch such as a thyristor, a solid-state relay (hereinafter referred to as SSR), or an electromagnetic relay having contacts to a load in series and provides a load driving instruction signal to turn ON and OFF the switch or the relay, to thereby control the operation of the load.
If the semiconductor switch short-circuits or if the relay contact melts, a current will flow to the load even if there is no input signal (load driving instruction signal). Namely, the conventional circuit has a danger that it may erroneously provide an output to the load although there is no input. Such circuit is not fail-safe, and therefore, is unemployable for devices that require a high degree of safety. To be fail-safe, the load driving circuits may employ an electromagnetic relay having special contacts (for example, carbon contacts) that never melt. This sort of contacts, however, is short in service life.
To secure fail-safe characteristics, another type of load driving circuits has been proposed (Japanese Unexamined Patent Publication Nos. 60-223445 and 60-227326 and U.S. Pat. No. 4,661,880). These circuits directly control a load driving switch circuit with an input signal (load driving instruction signal) and monitor the ON/OFF status of the switch circuit through a fail-safe monitor circuit.
Upon detecting electricity supplied to a load with no input signal, the monitor circuit forcibly breaks a primary power source, to surely prevent the most serious accident during the operation of the load.
Another type of load driving circuits connects an input signal to a power supply circuit of a load via an electrically isolated signal receiving system involving a transformer. According to this type, an AC input signal (load driving instruction signal) is amplified by an amplifier, and the amplified signal is supplied to a primary winding of the transformer so that a secondary winding thereof may generate an alternating current. The alternating current is converted by a rectifier diode into a direct current, which is supplied to the power supply circuit of the load.
This arrangement involves no semiconductor switches that may cause short-circuit failures nor has the problem of short service lives of electromagnetic relays, thereby ensuring fail-safe characteristics.
Even of this type, load driving circuits of large capacity for, for example, presses usually employ contact breaking mechanisms having relays for breaking a primary power source that supplies electricity to a load. Since the contact breaking mechanisms always have the problem of melt and wear, they are unsatisfactory in reliability.
According to the technique of indirectly driving a load through a transformer in response to an input signal, the load will generate a counter-electromotive force when the input signal is turned OFF, if the load is a DC electromagnetic valve or relay that is inductive. The counter-electromotive force produces a discharge current, which flows to a power supply circuit of the load through a rectifier diode. This results in causing a delay in stopping the load after the turning OFF of the input signal.
Some loads such as electromagnetic valves and relays show hysteresis that an input level for starting the loads differs from an input level for stopping the loads. These hysteresis loads continuously operate if an input level sufficient for maintaining the operation is supplied thereto after the start thereof. In spite of this phenomenon, the prior art continuously supplies the starting input level as it is to the loads, thereby wasting electricity.
Accordingly, an object of the first invention is to provide a fail-safe load driving circuit employing a non-contact breaking mechanism for breaking a primary power source. An object of the second invention is to provide a load driving circuit for supplying a high voltage to start an inductive load showing hysteresis that an operation stop voltage is lower than an operation start voltage and supplying a voltage that is slightly higher than the operation stop voltage during a steady-state operation, thereby lessening a delay in stopping the load after the turning OFF of an input signal. An object of the third invention is to provide a load driving circuit that is capable of saving electricity when driving a hysteresis load.